1. Field of the Invention
The present invention relates to a high-strength cross-laminated film, particularly a three-layer cross-laminated film with a foam core which is made by counter-rotating dies, and to a method of producing the laminated film.
In industry, a widespread need has arisen for the provision of extremely strong thermoplastic film webs which are possessed of a high degree of strength in different directions of the film webs, particularly when such film webs are employed in the manufacture of bags or the like. Moreover, in the production of various kinds of consumer products utilizing films of the type considered herein, limitations are encountered in the gauge reduction which can be effected, irrespective of the strength of the film, inasmuch as the feel and perception of a thin film by a consumer is frequently deemed to be undesirable. Consequently, from the standpoint of providing a product which is aesthetically and textually attractive to consumers, the employment of a foamed material for the center layer of a three-layer thermoplastic film material will substantially increase overall thickness of the film for a given amount of material, there creating the perception in a consumer of the presence of a thicker and stronger film, and with an attendant enhancement in the sales appeal of such a product.
Generally, a uniaxially oriented thermoplastic film, for example, a film which is constituted from a high density polyethylene, is extremely strong in one direction of orientation, for instance, in the machine direction of the film web, and very weak in another direction of the film. This renders the film susceptible to tearing along the direction of the orientation, but of a high strength and almost impossible to tear transversely of the direction of film orientation. In order to impart a thermoplastic film, particularly a high-strength laminated film, with an extremely strong film structure in various directions across the film, two layers of such a film can be laminated with the directions of orientation of the respective film layers being either crossed or extending in different directions of orientations. Each film layer imparts a high degree of strength to the laminate across its particular direction of orientation. However, it is generally desired in the laminated film structure that the two film layers not be rigidly adhered or tightly bonded, inasmuch as such a tight bonding or adherence causes any force encountered at the notch of a tear be concentrated within a very small area of the opposite bonded film layer, thereby resulting in a high stress and attendant film failure.
2. Discussion of the Prior Art
Various types of methods and apparatus have been developed in the thermoplastic film producing technology for the formation of cross-laminated thermoplastic films which, in general, possess a relatively high strength in different directions of film orientation. However, such films are usually expensive to manufacture in that they necessitate the utilization of complex apparatus and production methods.
Thus, in the known so-called Valeron process, Van Leer Plastics, an extremely strong laminated film is produced in that a uniaxially-oriented high density polyethylene film is extended through a tubular die, and thereafter slit and wound, as a consequence of which the primary orientation of the film becomes diagonal. Thereafter, when two layers of such films are superimposed and laminated and bonded together with a soft, extendable adhesive, with the axes of orientation of the film layers being crossed, the result is the formation of a high-strength cross-laminated thermoplastic film. However, this particular prior art process is expensive and, as a general premise, commercially uneconomical inasmuch as it necessitates the implementation of a multiplicity of complex manufacturing steps; moreover, it requires the use of a relatively heavy minimum film thickness due to the necessity of having to form multiple layers each of which must be of a reasonable thickness; and the resultant product is a flat sheet which, in order to form it into a bag, necessitates both back and end sealing, neither of which can be a heat seal inasmuch as this would tend to destroy the orientation of the film layers.
Sharps, Jr. U.S. Pat. No. 4,496,413 relates to a method of forming a blocked cross-plied polymer film in which there is no bonding between the film layers through the interposition of a soft, foamed adhesive material as in the present invention.
Rasmussen U.S. Pat. No. 3,471,353 discloses a method and apparatus for bonding oriented flexible sheet materials having the different directions of orientation, and in which the film layers are united through the interposition of a vaporized solvent and the subsequent utilization of heat. This to some extent provides a rigidly bonded film which is subject to the disadvantages described hereinabove encountered during the notching of such laminated film sheets. Moreover, the application of heat frequently destroys the effect of the cross-lamination of the film web.
Other apparatus and methods for producing cross-laminated thermoplastic film webs are disclosed in Sudo U.S. Pat. No. 3,853,661; Reifenhauser, et al. U.S. Pat. No. 3,926,706; Dyer U.S. Pat. No. 3,342,657; Suzuki U.S. Pat. No. 4,011,1281; Rasmussen U.S. Pat. Nos. 4,039,364 and 4,420,451; and Papesh, et al. U.S. Pat. No. 3,649,143. However, none of these publications provide for a lamination with a soft, extendable adhesive as contemplated by the present invention, and with the utilization of counter-rotating extrusion dies.
Pursuant to a more recent development by the present inventor, as set forth in copending U.S. Pat. No. Appln. No. 145,011, filed on Jan. 19, 1988, entitled HIGH-STRENGTH CROSS-LAMINATED FILM MADE BY COUNTER-ROTATING DIES, which is commonly assigned to the assignee of this application, there is contemplated the provision of a high-strength cross-laminated tubular film web which is produced by counter-rotating dies; the latter of which will allow for the orientation of each extruded film layer in a different direction so as to form a cross-laminate, and whereby, concurrently with the extrusion of the molten polymer layers, there is interposed a soft extendable adhesive to provide a weak bonding or adherence therebetween. This will cause the formation of a high-strength, cross-laminated film which, when one film layer of the cross-laminated film is caused to tear along the direction of orientation thereof, the force of the tear is transferred to and dispersed over a relatively large area of the adjacent film layer, the orientation of which is extremely strong in that particular direction, thereby inhibiting any continued tearing of the laminated film material.
The utilization of a counter-rotating die to form the cross-laminated film web is of particular advantage in that the orientation of the film during extrusion thereof is generally twisted so as to produce a diagonal orientation. Although this can be carried out by rotating an extrusion die while extruding the thermoplastic material and concurrently restraining the upper end of the resultant extruded film bubble to prevent it from rotating, this is difficult to carry out in actual practice inasmuch as the film would be subjected to a substantial torque tending to cause the film bubble to collapse. However, upon constructing the tubular extrusion die as a hollow member, and arranging a further counter-rotating die within the first-mentioned die, this causes the interior film layer to be diagonally oriented in an opposite direction to the outer film layer, and upon joining of the film layers into a laminate, the torque of the two layers reacts against each other so as not to impart any net torque on the laminate film bubble.
However, notwithstanding the advantages over the prior art which is attained through the intermediary of the cross-laminated tubular film web having a center layer of a soft extendable adhesive, this still may not fully meet consumer needs through the provision of a cross-laminated film possessing all of the desired physical properties as to strength and resistance to tearing. The reason for this may be traced back to an inherent desire in a consumer to be able to perceive the film product as being heavy and substantial to touch and in handling. This creates a distinct impression of the film being heavy or thick and possessed of a high degree of durability and inherent strength.